Plant seeds contain a number of different tissues including the embryo and cotyledons that are usually encased in a layer of thickened and lignified tissue referred to as a seed coat. In general, seed coats contain a significant portion of the total undigestible fiber content of plant seeds. Plant seeds with reduced fiber content provide many advantages for use as feed products. Thus, alteration of the seed coat composition, as well as alteration of the composition of the other tissues of the seed for reduced fiber content, can provide an improvement for plant seeds currently used for feed.
The seed coat provides a mechanical barrier that protects the seed prior to germination and allows the seed to remain dormant or withstand mechanical challenges. Some plant species have extremely strong seed coats that can withstand significant mechanical and environmental insult. Other plant species have thinner seed coats that offer a limited degree of protection from mechanical damage. The nature of the seed coat is determined genetically and is typically correlated with the biology and ecology of the plant species.
In many crop species of commercial interest, a thick seed coat is generally undesirable since the seed coat tends to contain a high level of undigestible fiber and is often a waste product upon processing of the seed for oil, meal or other products. The seed coat contributes a significant portion of the fiber content to plant seed meals. Thus, reduction of the seed coat is an important goal for crop improvement in many crop species. However, the importance of the seed coat for the protection of the seed itself dictates that any reduction in seed coat still allow for the protection of the seed from injury or damage during seed harvesting, processing, or planting of seed. Accordingly, a balance between seed coat size and composition and the mechanical barrier function the seed coat provides must be achieved.
The composition of the seed coat (or hull) is also a consideration for improvement in many crop species, particularly those that are used for feed. Fiber content of meals derived from plant seed is an important consideration for formulation of rations. Fiber levels of feed products must be carefully maintained for many applications since high levels of dietary fiber are associated with poor utilization of the meal and, in some cases, limits the utility of the meal. The plant cell wall constitutes the majority of dietary fiber and this fiber is composed of a relatively limited number of starting compounds arranged in a large number of different final products. The matrix of the cell walls contains most of the fiber component and this fiber is composed of various polymers in both covalent and non-covalent bonds.
Examples of covalent “fiber” bonds include esterified cross-linked sugar residues such as those found in pectins and non-cellulose polysaccharides and cross-linked lignin and extensin molecules. Non-covalent “fiber” bonds include associations in cellulose fibers and Ca++ ion bridges between pectins. The cell walls and associated “fiber” component of Brassica seeds include primary cell walls and some specialized types. Seeds consist of three main parts: cotyledons, embryo axis and reserve tissues. Cotyledons are generally thin-walled parenchema cells in contrast to the pericarp and testa that contain thickened, lignified and suberized cell walls embedded with various undigestible compounds. Accordingly, the dehulling approach has an obvious advantage in the physical separation of the portions of the seed high in fiber. Although the hull is a relatively small portion of the seed on a weight basis, it does contain a significant amount of the fiber content of seeds. Further reductions in fiber content would be desirable.
Unfortunately, the precise biochemical composition of the fiber component of each of the cell types in plants has not been carried out. Therefore, all “fiber” content figures tend to be generalizations and may not accurately reflect the actual composition of the fiber component. At the most simple and general level, plant cell walls or fiber are composed primarily of four complex compounds. These are cellulose, non-cellulose polysaccharides, proteins and phenolic compounds or lignins. Cellulose is a simple compound comprised of repeating glucose residues, however, the actual supramolecular structure of the molecule is complex. Non-cellulose polysaccharides are comprised of acid pectic polysaccharides, hemicelluloses and various polymers of structurally distinct sugars. There are a number of protein components to the fiber, the largest portion of which is extensin, a unique protein that forms a backbone for the further cross-linking of many compounds. Lignin, of course, represents the primary phenolic component. For the most part, none of these components are effectively utilized by monogastric animals in diets.
Fiber content in seeds and seed meal is generally expressed as crude fiber, acid detergent fiber or neutral detergent fiber. Crude fiber (CF) typically includes lignin, cellulosic, hemicellulosic and pectin fractions of the seed. Acid detergent fiber (ADF) typically includes cellulosic and acid stable lignin fractions, while neutral detergent fiber (NDF) tends to represent lignin, cellulose and hemicellulose components. Thus, the term “fiber” can mean many different chemical components. The methods for determination of the levels of these various fiber components are standardized according to AOAC (Association of Official Agricultural Chemists) methods.
The digestibility of seed meal is dependent on the composition of the fiber component. Some seed coats or hulls are highly lignified and generally resistant to degradation following ingestion while other seed coats may have a composition that allows easier degradation in the gut of an animal. Thus, a seed coat that is low in fiber is an important objective for crop improvement. Similarly, the composition of a seed coat will influence the processing of seed for seed products such as protein, starch or oil. Seed coats that are more amenable to processing are preferred. This may include seed coats with reduced level of pigments or seed coats with altered secondary metabolite composition.
Generally, Brassica oilseed crops are processed for oil and meal by the use of crushing techniques. Oil is extracted from seed following the disruption of the seed and the resultant solid material is referred to as meal. Typically, seed coats are found in the meal fraction. Although it is possible to remove the seed coat (dehull the seed) before processing, removal of the hull is an additional cost and leads to additional waste products.
In Brassica, there are numerous types of oilseed varieties, including high erucic acid, rapeseed and canola quality varieties. Canola quality varieties are the most predominant types of oilseed Brassica species grown for edible oil. Canola quality refers to a specific oil composition with reduced glucosinolate and erucic acid content providing a highly valuable edible oil. Brassica varieties that produce high levels of erucic acid are grown for industrial purposes and rapeseed lines are generally not used for edible oil production, except under certain instances or locations where the lower quality of rapeseed oil is tolerated by market conditions. Although rapeseed is widely grown, rapeseed oil does not command the premium seen for canola quality oil. Thus, canola quality varieties are of primary value to the industry.
Many Brassica species produce seed that typically has a dark seed coat, with a few species producing seed with a yellow seed coat. The normal black seed coat of canola quality oilseed B. napus imparts undesirable visual characteristics to both the oil and the meal of canola varieties upon typical processing of canola seed. Upon crushing of the canola seed, oil and meal fractions are isolated that are contaminated with seed coat or pigments found within the seed coat. The oil is dark during the initial stages of processing which makes it appear spoiled. In meal, the bits of black seed coat mixed with the light meal make it appear to be infested with insects. Thus a seed coat that is lighter in color can have advantages for the canola crushing industry.
Breeding canola quality Brassica seed with reduced seed coat has been an important objective for canola breeders. Some Brassica species have a seed coat that is yellow in color and it has been found that the yellow color is associated with a seed coat that is typically thinner and reduced in size. The yellow seed coat also appears to have reduced fiber and is likely more digestible due to the absence of certain pigments or secondary metabolites commonly associated with a dark, thicker, more lignified seed coat. Meal produced from yellow-seeded species of Brassica will typically have less fiber and provide a product that has, on a percentage basis, higher protein content, thus being more valuable. Accordingly, reduction of the size of the seed coat will carry many advantages. Thus, the development of Brassica napus species with a yellow seed coat is an important goal for the Brassica oilseed industry.
Studies have been carried out with the objectives to find new sources of genes encoding the formation of a yellow seed coat in Brassica, for example: Wu-JiangSheng et al., 1997, study on a new germplasm resource of a dominant gene controlling yellow seed coat in B. napus L., Journal of Huazhong Agricultural University, 16:1, 26-28; Wu-JiangSheng et al., 1998, a study on the inheritance of a yellow-seeded mutant of rapeseed (B. napus L.), Chinese Journal of Oil Crop Sciences 20:3, 6-9; Li-JiaNa et al., 1998, an initial study of the inheritance of seed color in yellow-seeded rapeseed (B. napus) lines with different genetic backgrounds, Chinese Journal of Oil Crop Sciences 20:4, 16-19. However, most of the traits identified are not suitable for simple introgression into canola quality Brassica breeding lines.
Other attempts to introduce a yellow seed coat into B. napus have included the introgression of the yellow-seeded trait from other Brassica species that are being developed into edible oilseed quality breeding lines. Examples of these studies include: Barcikowska et al., 1997, seed coat pigmentation—F2 yellow-seeded forms of B. juncea Coss X B. carinata Braun, Rosliny-Oleiste 18:1, 99-102; Meng-JinLing et al., 1998, the production of yellow-seeded B. napus (AACC) through crossing interspecific hybrids of B. campestris (AA) and B. carinata (BBCC) with B. napus, Euphytica, 103:3, 329-333; Qi-CunKou et al., 1996, studies on the transfer of yellow-seeded trait from B. carinata to B. napus, Jiangsu Journal of Agricultural Sciences 12:2, 23-28. Although it is possible to obtain yellow-seeded lines from these interspecific crosses, the resultant lines are often unstable with regards to the trait and stabilization and management of the trait during the breeding process often proves unreliable.
Accordingly, some experiments have been carried out to provide a means to stabilize the trait, e.g., Vyvadilova et al., 1999, the use of doubled haploids to stabilize yellow-seededness in oilseed rape (B. napus), Czech Journal of Genetics and Plant Breeding, 35:1, 7-9, but the ability to routinely stabilize and obtain yellow-seeded varieties is not predictable or conveniently accomplished.
Still other work has been conducted to identify molecular markers that co-segregate with the yellow-seeded trait as a means to more efficiently manage the production of yellow-seeded Brassica varieties. These include: Chen-BY et al., 1997, identification and chromosomal assignment of RAPD markers linked with a gene for seed color in a B. campestris-alboglabra addition line, Hereditas-Landskrona 126:2, 133-138; and Deynze-AE-van, et al., 1995, the identification of restriction fragment length polymorphisms linked to seed color genes in B. napus, Genome 38:3, 534-542.
Still other studies have attempted to select mutation in Brassica to impart the yellow seed color. WO98/49889 A1 teaches a method to select yellow-seeded characteristics from rapeseed lines through the use of microspore culture and selection of mutated lines. The resultant plants must be used for breeding into canola quality lines. This is a difficult and laborious process to carry out if the intent is to derive canola quality lines containing a yellow seed coat.
Despite all of these studies, a convenient source of a lower fiber, yellow-seeded trait in Brassica or a convenient means to manipulate the naturally occurring trait in other Brassica species has yet to be identified. In addition, the development of yellow-seeded varieties with low fiber content is a most preferred objective of many Brassica oilseed breeding programs. However, this has been difficult to accomplish to date. Thus, nearly all of the B. napus oilseed crops commercially grown still have the dark-seeded characteristic and only a few have varying degrees of yellow-seeded characteristics. The fiber content of conventional canola varieties has remained more or less constant despite these efforts towards the production of low fiber, yellow-seeded varieties. Thus, it remains an important objective for the Brassica oilseed industry to develop yellow-seeded canola varieties.
Development of a means to reduce fiber and manipulate seed coat color in related Brassica species, including members of the cruciferous family, can open the possibility of developing canola quality crop species from those species where seed coat color and characteristics are undesirable. Therefore, the ability to develop crops and, in particular, cruciferous crops, with reduced fiber and altered seed coats is an important element in the further development of new oilseed and meal crops.
As stated above, many factors control and/or influence whether a seed is dark or yellow. Proanthocyanidins (PA), also known as condensed tannins, are colorless flavonoid polymers that result from the condensation of flavan-3-ol units. In Arabidopsis, for example, PAs are found only in the seed coat where they confer a brown color to mature seeds, usually after oxidation. The aromatic aldehyde reagent, p-dimethylaminocinnamaldehyde (DMACA) specifically reacts with PA polymers, small oligomers, flavan-3,4-diols and flavan-3-ols to generate a deep purple-brown color.
Understanding of the mechanism and/or means to control the production and/or expression of PA would be a great benefit to seed production in general. The ability to alter and/or regulate the production of PA could assist in opening the possibility of producing crops, including cruciferous crops, with reduced fiber and altered seed coats. Such an ability would be an important element in the development of oilseed and meal crops.